While the 7970 is about as fast as a single 580. Heavily OC'ed it's 30% faster, tops, and in only a few games.

So obviously something is up here.

Something is up with your numbers. Stock for stock a 7970 is anywhere from 20-40% faster than a GTX 580 and overclocked that grows to 50-60%. All this is dependent on title and resolution but by no means are the two chips equal.